Subject: Re: [PATCH net-next 5/6] net: use core MTU range checking in virt drivers

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 19 Oct 2016 16:59:46 +0300

On Tue, Oct 18, 2016 at 10:33:32PM -0400, Jarod Wilson wrote:
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index fad84f3..4885a42 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1419,17 +1419,6 @@ static const struct ethtool_ops virtnet_ethtool_ops = {
>  	.set_settings = virtnet_set_settings,
>  };
>  
> -#define MIN_MTU 68
> -#define MAX_MTU 65535
> -
> -static int virtnet_change_mtu(struct net_device *dev, int new_mtu)
> -{
> -	if (new_mtu < MIN_MTU || new_mtu > MAX_MTU)
> -		return -EINVAL;
> -	dev->mtu = new_mtu;
> -	return 0;
> -}
> -
>  static const struct net_device_ops virtnet_netdev = {
>  	.ndo_open            = virtnet_open,
>  	.ndo_stop   	     = virtnet_close,
> @@ -1437,7 +1426,6 @@ static const struct net_device_ops virtnet_netdev = {
>  	.ndo_validate_addr   = eth_validate_addr,
>  	.ndo_set_mac_address = virtnet_set_mac_address,
>  	.ndo_set_rx_mode     = virtnet_set_rx_mode,
> -	.ndo_change_mtu	     = virtnet_change_mtu,
>  	.ndo_get_stats64     = virtnet_stats,
>  	.ndo_vlan_rx_add_vid = virtnet_vlan_rx_add_vid,
>  	.ndo_vlan_rx_kill_vid = virtnet_vlan_rx_kill_vid,
> @@ -1748,6 +1736,9 @@ static bool virtnet_validate_features(struct virtio_device *vdev)
>  	return true;
>  }
>  
> +#define MIN_MTU ETH_MIN_MTU
> +#define MAX_MTU 65535
> +

Do we need a common macro for this?

>  static int virtnet_probe(struct virtio_device *vdev)
>  {
>  	int i, err;
> @@ -1821,6 +1812,10 @@ static int virtnet_probe(struct virtio_device *vdev)
>  
>  	dev->vlan_features = dev->features;
>  
> +	/* MTU range: 68 - 65535 */
> +	dev->min_mtu = MIN_MTU;
> +	dev->max_mtu = MAX_MTU;
> +
>  	/* Configuration may specify what MAC to use.  Otherwise random. */
>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_MAC))
>  		virtio_cread_bytes(vdev,
> @@ -1875,8 +1870,10 @@ static int virtnet_probe(struct virtio_device *vdev)
>  		mtu = virtio_cread16(vdev,
>  				     offsetof(struct virtio_net_config,
>  					      mtu));
> -		if (virtnet_change_mtu(dev, mtu))
> +		if (mtu >= dev->min_mtu && mtu <= dev->max_mtu) {
> +			dev->mtu = mtu;
>  			__virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);

I think the logic is wrong here:

If mtu is legal, we set it but do not tell host.
If it's out of range we tell host we use it
but don't actually.

Should be the reverse.

> +		}
>  	}
>  	if (vi->any_header_sg)

-- 
MST